The present invention relates in general to light detectors and, in particular, to a new and useful solid state detector for detecting the ultraviolet radiation of a flame to determine whether the flame is on or off.
The safe operation of multiburner boilers is dependent upon instrumentation to verify the presence of flame at each burner. For certain gaseous fuels (i.e., natural gas, methane, hydrogen, CO, etc.) the byproduct of the flame which is usually sensed is the ultraviolet radiation (UV) which is emitted during combustion. Previous instrumentation to sense this UV consisted of photosensitive tubes which enclosed metal electrodes within a partially evacuated glass envelope. When the electrodes are connected across a source of electrical potential (typically 300-1000 volts) and a photon of light strikes the electrodes, the tube conducts current from one electrode to the other.
There are several shortcomings associated with tube type detectors.
Tube sensitivity and spectral response are difficult to control; therefore, tubes of like design may be unequal in their ability to monitor a flame.
The tubes reliability is low since they are fragile and age rapidly when exposed to heat or strong light. The life of a tube is limited by the rate of air infiltration through the glass wall and at the glass to metal seals of the electrodes, neither of which are perfect.
A high potential of several hundred volts is required to operate most such tubes. This potential is a safety hazard and poses unique material selection problems for transmission and termination components of the system.
The tubes are prone to fail in an unsafe mode. For example, small amounts of electrical leakage in the tube or supply voltage system will cause a "flame on" signal when no flame is present. Mechanical shock can also cause a false flame on signal due to electrode movement. Expensive and unreliable electro-mechanical shutters therefore must be used to block radiation to the cell in order to prove that they have not failed in an unsafe mode.
The tubes are becoming very expensive due to lower production as various UV measuring applications adopt solid state sensors.
Solid state silicon photocells with enhanced UV sensitivity have been tested for use as a flame detector (since 1977), but have failed. Failure is attributed to the fact that although the cells are sensitive to UV in the spectra of interest, their infrared response is many orders of magnitude greater than their UV response. Since a boiler outputs 10,000 times more IR (interference) than UV, the signal to noise ratio using silicon cells with the best available filters is too low to be acceptable.